The present terrestrial broadcast television system used in the United States is the NTSC (National Television Systems Committee) system which specifies coverage areas and signal quality. In general, the NTSC system divides the portion of the radio spectrum allotted for over-the-air television broadcasting into numbered 6 MHz bands or channels. In order to decrease interference between adjacent 6 MHz channels, the entire broadcast region has been divided into a number of geographical areas. Within each geographical area, only some of the total available channels are used.
However, the United States is in the process of formulating a standard for terrestrial broadcasting of high definition television. The current standards for broadcast HDTV make the system incompatible with the NTSC system. Since the NTSC system has been in operation for many years and a large number of private receivers are in place, it has already been decided by the Federal Communications Commission (FCC) that NTSC broadcast service will be maintained to existing receivers for a number of years to allow these receivers to be phased out.
Consequently, the FCC has decided that the NTSC system will remain in operation and HDTV broadcasts will be simulcast with NTSC broadcasts. The FCC has also decided that the HDTV system must be able to operate with minimal impact on the NTSC system.
The FCC's decisions translate into several major constraints for the new HDTV system. First, there will be no new allocation of the radio spectrum for the new service; secondly, the present channel bandwidth of 6 MHz will be retained. Finally, in order to minimize any impact on the NTSC system, HDTV transmitters will be required to operate with power levels 10-15 dB below the power level of current NTSC transmitters.
The aforementioned constraints mean that the "taboo" channels in a given area will have to be used for the new HDTV service. This latter fact coupled with the low transmission power requirement means that some receiver locations (particularly those at the edges of NTSC geographical areas) will receive both NTSC signals and HDTV signals in the same frequency band, a problem called "co-channel" interference. Co-channel interference may be so severe in some locations that it degrades HDTV system performance.
Several different digital high definition television systems are currently being developed by competing concerns. These systems share broadly similar characteristics in terms of video source coding. More particularly all of the proposed systems have roughly twice the horizontal and vertical resolution of the current NTSC standard. At the proposed resolution, the digitized picture requires a data transmission rate of 1-1.5 gigabits per second, a rate which would be impossible to achieve with present technology. Consequently, source coding is used to reduce the required data transmission rate to 15-20 megabits per second (Mb/s) and still yield good quality images. All of the proposed systems use the same type of source coding which is based on motion-compensated frame-to-frame prediction and the application of a block discrete cosine transform to reduce the correlation between neighboring pixels.
However, the compressed picture information must still be encoded further in order to ensure proper protection against channel errors. In order to achieve efficient spectral coding the proposed systems use known techniques for encoding the data. These systems may include, for example, n-ary quadrature amplitude modulated systems (n-QAM) in which both the amplitude and the phase of the signal relative to the carrier phase are modulated to produce n separate signal points. Alternatively an n-level vestigial sideband system (n-VSB) can be used.
The proposed systems differ in the way that video information is encoded before transmission over-the-air. One of the reasons for these differences is that each proposed system has given different degrees of importance to the various constraints discussed previously including: minimal impact on current NTSC coverage areas, robustness to NTSC interference, particularly co-channel interference, low transmitter power relative to NTSC, fixed bandwidth of 6 MHz and transmission in the so-called taboo channels.
Among these constraints, co-channel interference from neighboring NTSC stations is of particular importance. It is generally agreed that the HDTV service will be co-channel interference limited, rather than noise limited. Part of the reason is that noise is generated mostly in the receiver front-end and can be reduced by, among other things, appropriate design of the tuner, but the co-channel interference level depends on the location and power of the interfering station. However, when the existing NTSC receivers are eventually phased out, it is desirable that the HDTV system design be reconfigurable so that optimal transmission can be achieved in the absence of NTSC interference.
There are several conventional ways to deal with co-channel interference. One known method is to treat the interference as noise. This is what is effectively done in one known system by using a symbol interleaver between the encoder and the modulator transmit filter at the transmitter end and a deinterleaver at the receiver end between the demodulator receive filter and the decoder. Such a system is shown in FIG. 1. The source bit stream 100 is provided to an encoder 101, whose output, in turn, is provided to an interleaver 102 of conventional construction. The output of the interleaver 102 is, in turn, provided to the modulator and transmit filter 104. The output of the transmit filter 104 is sent over the broadcast channel 106 which is indicated by the dotted box 106. The signal plus the noise and interference arrives at the receive filter 108 and then is provided to the demodulator. The demdulator output is provided to the deinterleaver 110. The deinterleaver 110 restores the symbol stream but scrambles the noise and the interference. The data is then decoded by a decoder 112. This method has several advantages. One is that the interference becomes equivalent to added noise and, in fact, has a lesser impact on the probability of error than additive white Gaussian noise with the same noise power. A second advantage is that interleavers are usually part of conventional transmission systems and therefore the design of the rest of the transmission system, in particular the equalizer, is well understood.
However, there are also drawbacks to this method. For example, additional delay is introduced and, while the system will work well for situations where the carder to interference level is high compared to the carder to noise level, it will perform poorly in the presence of strong interference.
Another known system is called the "DigiCipher" system proposed by General Instruments Corporation (GI). In this system, a linear equalizer mitigates the effect of the NTSC interference. FIG. 2 shows a simplified receiver for use with this system, in which the received signal is y(t),200. The signal y(t) is translated to baseband frequency as represented schematically by multiplier 204. The resulting noisy baseband signal as received over the channel is provided to the receiver. The receiver comprises a matched filter 206, a symbol-rate sampler schematically represented by switches 208 and 212 driven at the rate t=1/T, a symbol-spaced equalizer 210 and a slicer 214. In practice, the receiver would not be implemented in this fashion, but the example illustrates the principles involved. The filter function C (e.sup.jw) is chosen so that the mean square error MSE) at the output of the filter is minimized. The error between the slicer input and the transmitted symbol is the sum of the residual intersymbol interference (ISI) and the noise. Therefore, the power spectrum of the error is given by: EQU .sigma..sub.2.sub.x.vertline. 1-S.sub.h (e.sup.jw).vertline..sup.2 +N(e.sup.jw)S.sub.h (e.sup.jw).vertline..sup.2 ( 1)
where .sigma..sup.2.sub.x is the signal power, S.sub.h (e.sup.jw) is the folded spectrum of the received pulse and N (e.sup.jw) is the power spectral density of the noise. The MSE is the integral of equation (1). It is minimized by setting C(e.sup.jw) as follows: ##EQU1##
If the interference has strong spectral components at given frequencies, then the equalizer will attenuate those frequencies at the expense of increased inter-symbol interference and enhanced noise. In general, minimized MSE equalization should work well except in case of strong NTSC interference.
Another method that has been proposed in the context of a known prior art system known as the Zenith/AT&T Digital Spectrum Compatible system. This system uses a form of precoding at the transmitter and comb filtering at the receiver to create spectral nulls which are positioned near the main NSTC spectral components. The basic system is schematically shown in FIG. 3.
The basic modulation configuration for this proposal is as shown in FIG. 1. However, a precoder 304 (FIG. 3 ) is inserted between the encoder 101 (in this case a Reed-Solomon encoder would be used) and a modulator 306 and operates in the digital domain. The input 300 to the precoder 304 is a four-level signal (4L). It is precoded with a modulo-4 adder 302 (a nonlinear operation) and a delay element 308. By choosing the delay element appropriately, spectral nulls can be positioned close to the locations of the NTSC video, color and audio carrier frequencies, thereby eliminating the steady-state portion of the interference. For a 4VSB modulated signal with the carrier on the band edge, the required delay corresponds to 12 symbols (indicated as 12 symbol delays, 12D). The output of the precoder is applied to the channel 312 via the modulator 306.
The operation perforated by the modulo-4 adder 302 can be reversed by a linear operation performed at the receiver. Specifically, the signal received from the channel 312 is applied, via demodulator 314, as a four level signal (4L) to a comb filter 313 comprised of a 12 symbol delay element 316 and a digital adder 318. The output of the comb filter is a 7 level signal, 7L, which is applied to a modulo-4 decoder 320 to produce a 4 level output signal. The portion of the interference that does not get rejected is then passed through a deinterleaver 110 as shown in FIG. 1.
The noise performance of this latter system is degraded by 3 dB when the co-channel interference is the major source of degradation. However, if co-channel interference is not a problem, as would be the case in most areas and in all areas when NTSC receivers are phased out, the receiver can be redesigned so that the exact inverse of the precoder can be used at the receiver with almost no noise performance loss and simply a doubling of the symbol error rate.
This redesigned system is shown in FIG. 4. Most of the system is equivalent to the system shown in FIG. 3 and equivalent elements have been given similar numerals (for example, modulo-4 adder element 302 in FIG. 3 corresponds to modulo-4 adder element 402 in FIG. 4). The receiver has been modified by adding a slicer 422 and a post-coder 424 after the demodulator 414. Post-coder 424 is comprised of a 12 symbol delay element 426 and a modulo-4 adder 428.
One drawback of this system is that it appears to require hard-decision decoding (the slicer 422 is placed before the postcoder 424) in the case where a modulo-4 postcoder is used. This method is therefore not compatible with maximum likelihood soft decision decoding and cannot be expected to produce optimum results. Further, the design does not use a high-rate signal sequence coding scheme, such as trellis coding; instead error control is implemented with a single stage Reed-Solomon code. Therefore, even after NSTC co-channel interference is abated, the redesigned system cannot be made optimal.
The proposed design also takes advantage of the strong attenuation of existing NTSC receiver filters near the low end of the RF band by generating a pilot tone at these frequencies to aid carrier recovery. As discussed below, this pilot tone may interfere with the NTSC system due to non-linearities in the transmitter and receiver components.
Another system was proposed by the Advanced Television Research Consortium for the ADTV system. In this latter system, the data is organized in two streams and transmitted in a multichannel mode. Each stream is modulated on its carrier with a 32-QAM scheme (optionally a 16-QAM scheme). The streams are separated and arranged so that part of the signal is transmitted in a band below the NTSC career frequency and pan of the signal is transmitted in a band located above the NTSC carrier frequency. A null occurs at the NTSC carrier frequency; consequently, no signal power is transmitted near this frequency. The power spectral density of the lower frequency band or the "high priority career" is higher by 5 dB compared to the higher frequency band or "standard priority career". This power difference ensures that the carrier-to-noise threshold of the high-priority channel is 5 dB lower than the career-to-noise threshold of the standard priority channel. The high priority data represents one fifth of the total power. This data transmission scheme can be viewed as a particular case of data transmission using a modulation scheme with an unequal constellation. As transmission schemes of this type are known, the scheme will not be discussed further herein.
As with the previous proposed design, this latter design relies on the fact that the high priority channel occupies a band which is normally strongly attenuated by the Nyquist filters of NTSC receivers, and the idea is that this high power digital signal will cause very little interference to NTSC receivers, while at the same time avoiding the NTSC video carrier frequency that is the frequency component most detrimental to the HDTV signal. However, it is not known whether all NSTC receivers will be immune to an increased interference level near DC. Another aspect that could cause problems is the presence of nonlinearities either at the transmitter or the receiver. One example of nonlinear element is the Traveling Wave Tube (TWT) amplifier that is often used in the RF stage of the transmitter. The transfer characteristic of a traveling wave tube near the saturation region is nonlinear and operation in this region would lead to the creation of both in-band and out-of-band intermodulation products between the two carriers.
Another problem may arise from the fact that timing recovery is derived from the high priority channel. The narrowband characteristic of the high priority channel may result in a slow acquisition time compared to a signal with basic sampling rate 5 times higher.
Further, this latter system requires the signal to be formatted at the transmitter. Consequently, when NTSC interference is no longer a problem, the signal will be sub-optimal because it wastes bandwidth at the NTSC carrier null, but at that time the number of existing receivers will be so numerous that it will not be possible to change the signal formatting.
For reference, the reported performance for three proposed systems discussed above is given in Table 1.
TABLE 1 ______________________________________ System Modulation C/I ______________________________________ GI 32-QAM 7 dB 16-QAM 2 dB Zenith 4-VSB 0 dB 2-VSB -6 dB ATRC SP 32-QAM -2 dB HP 32-QAM -6 dB ______________________________________